The present invention relates generally to a brake control system, and more particularly, to a system for estimating the brake-fluid pressure.
In recent years, vehicle-mounted braking devices have various functions added. Such braking devices are known, for example, in the form of an antiskid braking system for achieving shortening of the braking distance while maintaining the vehicle behavior stable by preventing wheel lock, a traction control system for preventing spin of driving wheels during vehicle acceleration and the like, a brake assist device constructed so that when the brake-fluid pressure operated by a driver is insufficient, this insufficiency of brake-fluid pressure is supplied to a wheel cylinder, and a vehicle-stability control system for securing the running stability of the vehicle by providing a braking force to the wheels regardless of presence of brake operation by a driver so as to remove oversteer or understeer of the vehicle.
With the above braking devices, accurate grasp of the fluid pressure within the wheel cylinder and that produced by operation of a brake pedal will allow more precise control of a driving time or the like of a control valve for controlling the fluid pressure, leading to further enhanced control accuracy of the braking devices. One approach is to provide a fluid-pressure sensor to a master cylinder or a wheel cylinder so as to measure the fluid pressure within the master cylinder or the wheel cylinder. However, this approach involves an increase in manufacturing cost of the device.
Due to such problem of manufacturing cost, another approach is proposed wherein only the fluid pressure within the master cylinder is measured by the fluid-pressure sensor, and calculation is made as to how this pressure varies in accordance with a fluid-pressure model of the braking device to estimate the fluid pressure within the wheel cylinder. Specifically, a variation in the fluid pressure calculated in accordance with a drive signal for the braking device is added to an estimated fluid pressure (previous value) within the wheel cylinder obtained using the fluid pressure within the master cylinder, thus estimating a current value of the fluid pressure within the wheel cylinder.
However, an error can occur in the estimated current value. From a study of the cause of occurrence of the error made by the inventor, it was found that the error occurred depending on the operation responsivity of a solenoid valve. Specifically, as is well known, in connection with fulfillment of brake-fluid pressure control through the solenoid valve, means for executing low-cost and high-accuracy control include output of a duty control signal to the ON-OFF solenoid valve. The solenoid valve comprises an actuating member such as a spool. Then, it was found that due to its mass and operation resistance, the actuating member could not be driven when output of the duty control signal for a drive signal lasts a short time.
Specifically, when outputting to the solenoid valve control signals for operation of openingxe2x86x92closingxe2x86x92opening and that the closing signal has an output time extremely short, i.e. when outputting a control signal having an opening duty ratio extremely high or a closing duty ratio extremely low, the actuating member such as a spool cannot be actuated and maintained in the open state due to the above operation characteristics. In this case, though commands for operation of openingxe2x86x92closingxe2x86x92opening are provided by the control signals, the solenoid valve is actuated in a way of openingxe2x86x92openingxe2x86x92opening.
Therefore, when the solenoid valve includes a valve for increasing the fluid pressure within the wheel cylinder, an estimated value of the fluid pressure to be obtained in accordance with the drive signal is obtained by calculating a xe2x80x9cclosingxe2x80x9d part of operation of openingxe2x86x92closingxe2x86x92opening. However, since actual operation of the solenoid valve is maintained in the xe2x80x9copenxe2x80x9d state during a command for xe2x80x9cclosingxe2x80x9d operation as well, brake fluid is supplied to the wheel cylinder, leading to the actual fluid pressure higher than the estimated value.
On the other hand, when the solenoid valve includes a valve for decreasing the fluid pressure within the wheel cylinder, estimation is carried out assuming that a pressure decrease is not effective for a xe2x80x9cclosingxe2x80x9d part of operation of openingxe2x86x92closingxe2x86x92opening. However, since actual operation of the solenoid valve is maintained in the xe2x80x9copenxe2x80x9d state during a command for xe2x80x9cclosingxe2x80x9d operation as well, the actual fluid pressure is lower than the estimated value.
It is, therefore, an object of the present invention to provide a system for estimating the brake-fluid pressure, which contributes to an enhancement in estimation accuracy of the brake-fluid pressure within the wheel cylinder.
The present invention provides generally a brake control system for a motor vehicle, which comprises: a brake actuator actuated by a fluid pressure; a pressure control valve which adjusts the fluid pressure within the brake actuator, the pressure control valve selectively carrying out operation for supplying fluid to the brake actuator, operation for discharging fluid from the brake actuator, and operation for retaining fluid in the brake actuator; and an ECU which controls the pressure control valve, the ECU providing to the pressure control valve a first control signal including a duty-ratio signal comprising a valve opening drive signal as a supplying operation signal and a retaining operation signal upon the supplying operation of the pressure control valve, the ECU comprising an estimate part which is so constructed as to: totalize an influent amount of fluid in the brake actuator in accordance with an output time of the valve opening drive signal of the first control signal and a supply amount per unit time of fluid in the pressure control valve; estimate an amount of fluid in the brake actuator in accordance with the influent amount as totalized; estimate a value of the fluid pressure within the brake actuator in accordance with the amount of fluid as estimated; and carry out correction for obtaining a supply amount of fluid in the brake actuator when a time of the retaining operation performed by the first control signal is smaller than a predetermined value, the correction being performed by adding a correction value corresponding to the time to the output time of the valve opening drive signal of the first control signal.